5-15Sylphon cell contracts; as pressure decreases, it expands.As the Sylphon cell expands and contracts, itcommunicates motion to an indicating pointer on agraduated scale.Figure 5-2.Aneroid barometer.The aneroid barometer can be read in inches ofmercury and in millibars (mb). Both inches and millibarsare measurements of the weight of the atmosphere at agiven time and point. The average atmospheric pressureat sea level is 29.92 inches or 1,013.2 millibars. Figure5-3 shows the comparative readings on the inch andmillibar scales.The aneroid barometer normally can be read nocloser than 0.01 inch. Aneroid barometers are thestandard pressure-indicating instrument aboard ship andthe type of barometer that the POOW will encountermost frequently. Barometers are normally checked(calibrated) yearly by the quartermaster division,following the preventive maintenance system (PMS)procedures. Another important forecasting tool is thewind speed and direction indicator.Figure 5-3.Inches and millibars.Measuring Wind Speed and Direction.Aninstalled anemometer is shown in figure 5-4. It is aninstrument fixed somewhere aloft, usually at themasthead of the ship. The wind blows on a propellerattached to one end of a wind vane that pivots. Thewhirling propeller revolves a spindle, communicatingwith a synchro repeater on the pilothouse or chart housebulkhead. Figure 5-5 shows one type of synchrorepeater.The upper dial of the repeater is graduated in 10-degree intervals and shows the relative direction fromwhich the wind is blowing. In this illustration thedirection is about 287 . The lower dial indicates therelative wind speed (true wind speed when the ship isstationary). The wind-speed dial in the illustration showsabout 87 knots. This reading means that the forceexerted by 87 knots of wind is whirling the anemometerpropeller.When you use an installed anemometer, alwayscompare the readings observed with the wind conditionsas they appear outside. If two anemometers are installed,ensure that the windward anemometer is used.